dt-bindings: Add doc about rk3308 General Register Files

RK3308 GRF is divided into four sections: GRF, SGRF,
DETECTGRF, COREGRF. This patch add documentation for
it.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191021084555.28356-1-andy.yan@rock-chips.com


Signed-off-by: Heiko Stuebner <heiko@sntech.de>